1. Field of the Invention
This invention relates generally to optical communication and, more particularly to an optical network unit for use in switched networks.
2. Description of the Prior Art
There are two general classes of optical communication network architectures--broadcast and switched. In a broadcast network architecture, each fiber line from the Central Office (CO) feeds a corresponding large group of subscribers. Specifically, all of the optical information carried by a fiber line is presented to each subscriber's receiver or Optical Network Unit with Time Division Multiplexing (TDM) being used to distinguish the channels.
It has, for example, been proposed to utilize a broadcast structure in a Passive Optical Network (PON), an optical fiber network in which there are no active components between the central office (CO) and the subscriber's terminal equipment. The passive splitter-based network disclosed therein utilizes a "double star" broadcast structure in which fibers comprising the first star leave the CO in all directions and deliver optical information to the secondary stars. Each secondary star or Remote Node contains a star coupler which further distributes all of the optical information to either another level of stars or to Optical Network Units (ONUs) in the immediate locale. In all, each fiber line from the CO will feed as many as 128 subscribers.
It will be readily appreciated by those skilled in the art that since every subscriber to a network using a broadcast structure receives all the information destined for all subscribers, each subscriber's receiver or ONU must continually or periodically monitor the bus for network management. That is, the subscriber may have to respond to "ringing" signals, requests for status monitoring, interrogations, and the like, that are unscheduled. Consequently, the ONU can never be entirely "asleep" at any time, but rather must always be functioning at some marginal level at all times, even in the quiescent (or "Q") state. As a result, the ratio of the "on" power consumption to quiescent power consumption, P.sub.ON /P.sub.Q, is not very high, with typical values ranging from 2-6. It is therefore necessary to provide each ONU with back-up batteries having an energy storage capacity comparable to full operation for eight hours, even for an inactive network, so as to avoid an interruption in service in the event of a power failure.
The other general class of optical network structures is the switched network structure. In a switched network, the ONU/receiver associated with a particular subscriber receives only the optical information intended specifically for it. A PON system utilizing a switched configuration, for example, is proposed in U.S. patent application Ser. No. 08/029,724 filed on Mar. 11, 1993, entitled OPTICAL NETWORK BASED ON REMOTE INTERROGATION OF TERMINAL EQUIPMENT and assigned to the assignee herein, AT&T Corporation. In the aforementioned application, the disclosure of which is expressly incorporated herein by reference, wavelength-division-multiplexing (WDM) techniques are utilized in which the signals are split at the RN based on the wavelength of light, with light of a given wavelength being directed to a specific ONU.
It would be desirable to provide an ONU configured to exploit one of the properties of a switched optical network vs. a broadcast optical network. Specifically, in contrast to a broadcast optical network, the mere presence or absence of light in a fiber feeding a switched network ONU may be characterized as a "bit" of information, since the presence of light signifies that a particular ONU is being addressed. This characterization is, in fact, true of all switched networks, not just optical networks. Accordingly, it should be understood that the ONU of the present invention may also be employed in hybrid systems in which the signals are transmitted both by wire and fiber.
More particularly, it would be desirable to provide an optical network unit which may be readily switched from a power conserving quiescent mode to a normal operating mode when the presence of light is detected and in which the need for the ONU to detect light at the line rate or to employ special filtering techniques is obviated when responding to a "wake-up" call.